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Author Topic:   305 compression test
bizmans
Dirt Full Roller

Total posts: 28
posted January 26, 2005 06:38 PM  
can anyone tell me what is a good range for the compression test psi for a chevy 305 motor I belive mine is way low can you tell me if I am right. I am getting around 90 psi per cyl one was around 80 is time to rebuild or go to another motor I am trying to run purstock and before I fix it up I need to know if it is a good or bad motor.

petordragon
Dirt Maniac

Total posts: 167
posted January 26, 2005 08:20 PM  
squirt a little bit of oil into the cylinder bieng tested if psi increases rings are bad 10 psi diff between cylinders is not a good sign either sounds like a tired motor.

bizmans
Dirt Full Roller

Total posts: 28
posted January 26, 2005 08:28 PM  
what is an expected psi for this motor

bizmans
Dirt Full Roller

Total posts: 28
posted January 26, 2005 08:51 PM  
tested them 1 of them went up 10 psi all the others stayed the same

ruggs66
Dirt Maniac

Total posts: 193
posted January 27, 2005 07:52 AM  
if you are going to rebuild the engine the compression will be higher with new rings & a valve job done to the heads.

90 psi is low & I wouldn't race it like that.
But if its not Knocking it should be a good engine to rebuild.

It should go up to at least 120-150 psi when freshened.

bizmans
Dirt Full Roller

Total posts: 28
posted January 27, 2005 08:05 AM  
would you rebuild it or go to a 350 which is allowed for that year

ruggs66
Dirt Maniac

Total posts: 193
posted January 27, 2005 08:11 AM  
I would build the biggest engine that is legal to run.

It will be hard to keep up running a 305 if most guys in your class have 350s.


ruggs66
Dirt Maniac

Total posts: 193
posted January 27, 2005 08:14 AM  
But if you can't afford to buy & build a 350 I would rebuild the 305 & run that just to have some fun.

A 305 is better than watching from the stands!

bizmans
Dirt Full Roller

Total posts: 28
posted January 27, 2005 10:29 AM  
thanks that is what I will do. what compression should a 350 have? I will try to buy a truck 69-71 with the high compression but will test it first.

Stevei2i0
Dirt Full Roller

Total posts: 54
posted January 28, 2005 08:44 PM  
hey dude get you a 40 over dish top 305 add a 488 lift cam rhoads lifters an you got to have 53cc head to work with the 40 over dish pistons finish 5 in points out of 23 cars like i did 350 are good but 305s will run **** im running a 283 this season


bizmans
Dirt Full Roller

Total posts: 28
posted January 29, 2005 04:06 PM  
after all this **** and almost finishing the engine removal I found out the compression gage is not working right I bought a new one and I am getting and average of 125 psi per cylinder is that the norm for this motor

ruggs66
Dirt Maniac

Total posts: 193
posted January 30, 2005 08:32 PM  
what year engine is it?

125 is not super great but thats a used engine so its not bad..... but it should get higher when you rebuild it.

should be a good engine just to rebuild.

bizmans
Dirt Full Roller

Total posts: 28
posted January 30, 2005 09:24 PM  
it is a 1976 motor I all so tested it when it was cold so I assume when it is warm it might be higher

leapinlizard
Dirt Freak

Total posts: 402
posted January 31, 2005 08:42 AM  
While you can't put a specific number on cranking copression numbers due to all the variables involved such as, Barometric pressure, altitude, humidity, throttle angle ect..., You can assume that 125-180 pounds is normal. Generally your looking for trends in numbers. Two adjacent numbers that are lower indicate a head gasket while 1 really low one might be rings or valves. Oil in the chamber will raise overall but if a large change ocurrs then rings seal is indicated. Best of all, keep track of you numbers and they will give you a good base line graph to track you engines condition so you may refresh before costly repairs.

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